There is out there a "real" world of people, events and things. And there is also a world of words and statements. Our language, however, does not always reflect accurately the "real" world. The extensional devices considered here are designed to bring language to reflect more clearly the way in which science tells us the "real" world exists. Put differently, the extensional devices are designed to get us to talk about the world the way it is, and not the way our linguistic habits often lead us to believe it is.
1. Index. The index serves to emphasize the principle of non-identity, that no two things are ever the same. Politician1 is not politician2, husband1 is not husband 2.
2. Date. The date reminds us that everything is in a constant state of change, that nothing remains the same from one minute to the next. Mother(1958) is not mother(1998), Brother(1972) is not brother(1990).
3. Etc. The etc. emphasizes the principle of non-allness, that we can never know all or say all about anything. Whatever we say about something we have inevitably left something out. We could spend our entire lives listing the characteristics of another person, for example, but if our list were an accurate one it would have to contain an etc.
The initial reaction to these devices is that they are obvious. We all know that no two things are identical, that everything is always changing, and that knowledge is always incomplete. This, we may say, is basic philosophy.
These devices are simple and obvious. Yet, they are at the same time difficult and elusive. It is easy to realize intellectually that the devices are meaningful and worthwhile but most difficult to internalize them to the point where we act in accordance with them. They are, however, so important that we must try to make them a part of us.